Interface Engineering And Performance Optimization Of Qraphene/silicon Solar Cells

Graphene(Gr)is an important two-dimensional material,which has superior optical and electrical properties including high transparency and high carrier mobility.Therefore,graphene has attracted research interests around the world.The properties of graphene also make it to be an ideal material for photovoltaic applications.By combining graphene thin film and n-type silicon,graphene/silicon(Gr/Si)solar cells can be facilitated,which can simplify the fabrication process and reduce cost of the solar cells.However,by now,efficiency and stability of the Gr/Si solar cells are still low,which can not meet the need of practical application.In this thesis,a systematic study on the topic of interface engineering of Gr/Si solar cells has been performed.I composed several kinds of device structures to solve the major issues of Gr/Si solar cells.Some important results are summarized as follows:1)By introducing an ultrathin(0.5 nm)LiF interlayer in-between Si and Al electrode,Schottky barrier height and contact resistance of Si/Al were decreased,which suppresses carrier recombination at the rear surface and reduces series resistance of the solar cells.Efficiency of the Gr/Si solar cells with a LiF interlayer was 6.25%(pristine)and 10.61%(doped).2)A WO3 interlayer was deposited at the Gr/Si interface by thermal evaporation.The Gr/WO3/Si solar cell was found to be a quasi pn junction solar cell,the rectifying characteristics of which are similar with conventional pn junction solar cells.Such kind of solar cells have advantages including less carrier recombination at the interface and larger built-in field,which is superior to the normal Schottky junction solar cells.An efficiency as high as 10.59%was obtained by the introduction of the WO3 interlayer,which was higher than the chemical-doping-free Gr/Si solar cells in literatures.3)A room-temperature process was employed for the fabrication of Gr/HTL/Si quasi Schottky junction solar cells,in which Spiro-OMeTAD was selected as the interlayer(or hole-transporting layer,HTL).Here,the Spiro-OMeTAD interlayer has multiple functions including blocking electrons,reducing carrier recombination and enhancing effective Schottky barrier of the solar cells.The resultant efficiency was 13.02%,which is much higher than referential Gr/Si solar cells(5.95%)and other chemical-doping-free Gr/Si solar cells reported before.XPS results indicated the Gr/PMMA film serves as a protective coating of the Spiro-OMeTAD thin film,which prevents the invasion of O2 and H2O from ambient.As a result,the Gr/Spiro-OMeTAD/Si solar cells exhibited satisfactory stability,efficiency of which was maintained larger than 11%for over 4 months.These developments in both efficiency and stability reveal the great advantage of solar cells compromising the structure of Gr/HTL/Si,which could be encouraging for practical application of Gr/Si solar cells.4)High-performance Gr/n-Si solar cells were developed by exploring an illumination-induced hole doping in Gr/P3HT/n-Si solar cells.The p-type doping increased the work function and conductivity of the Gr layer,which,in turn,enhanced Schottky barrier height of the solar cells.Efficiency of the solar cells was improved by more than six times to 6.78%in assist with the photoinduced doping.Dual-side doping of Gr and antireflection techniques were further employed,which yielded an efficiency of 12.95%with enhanced stability.The introduction of the photoinduced doping is benefit to improving efficiency and stability of Gr/Si solar cells.